Overload sensing device employing independently operated thermal and magnetic trip means



United States Patent 3,201,543 OVERLOAD SENSING DEVICE EMPLOYING IN- DEPENDENTLY OPERATED THERMAL AND MAGNETIC TRIP MEANS James H. Leonard, Cedar Rapids, Iowa, assignor to I-T-E Circuit Breaker Company, Philadelphia, Pa., a corporation of Pennsylvania Filed Apr. 4, 1961, Ser. No. 100,734 14 Claims. (Cl. 200-88) My invention relates to an overload current sensing device and more particularly to an overload sensing current device employed in a circuit breaker being used for AC. interruption and having independently operated thermal and magnetic trip means where the overload sensing device acts to shunt the thermal trip means during severe overload conditions.

Molded case circuit breakers presently in use are provided with a time delay trip unit usually in the form of a bimetallic element and an instantaneous trip element which is in the form of a yoke armature combination. Upon the occurrence of an over current condition the deflection of the bimetal will be effective to unlatch the circuit breaker cradle mechanism and upon the occurrence of a fault current condition the magnetic yoke will be sufii-ciently energized to attract its armature thereby resulting in an unlatching of the breaker cradle mechanism in order to effect contact separation.

It has been found that the bimetallic element will be permanently deformed if subjected to excessive currents. Some circuit breakers of the prior art have included circuits which shunt the bus current so as to bypass the bimetal upon the actuation of the instantaneous trip-ping. In this way the bimetal is protected from excessive current simultaneously wit-h the movement of the armature towards its associated magnetic yoke responsive to the severe fault current. Since the armature has a substantial mass, its operating time is not instantaneous thereby causing the bimetallic element to draw this severe overload current during the time interval in which the armature is being operated.

It has also been found that upon the occurrence of a severe fault current condition the bimetal shunt circuit is not actuated by the instantaneous trip unit in a time interval which is sufiiciently short so as to prevent burnout or permanent distortion due to excessive heating in the case of a directly heated bimetal. The problem is particularly severe in circuit breakers having a low continuous current rating hence a high resistance bimetal.

One solution to this problem is that of providing a shunting circuit which is actuated by the bimetal itself. This bimetal shunt circuit is set forth in U.S. application No. 740,041, filed June 5, 1958, entitled Bimetal Actuated Bimetal Shunt by J. H. Leonard and assigned to the assi-gnee of the instant invention, now US. Patent No. 2,989,605 issued June 20, 1961. In the circuit breaker of the above mentioned application the free end of the bimetal carries a contact which upon sufficient deflection of the bimet al engages a contact in a shunt circuit which is completed When the two contacts are in engagement. The shunt circuit does not engage the contact of the bimetal during normal heating of the bimetal but it is engaged when there is overheating at a time before burnout or distortion take place. Thus the shunt does not carry current for an extended period of time but merely for some time less than it takes the circuit breaker to open after experiencing a short'oircuit or severe fault condition.

Under severe fault current conditions the rapid movement of the bimetallic element towards the shunt contact will cause the shunt member to rebound as a result of 3,201,543 Patented Aug. 17, 1965 this impact thereby separating the cooperating contacts. This contact separation has often resulted in excessive arcing between the shunt circuit contacts leading to a high resistance junction which introduces a high imped ance into the shunt circuit. In order to prevent rebounding of this nature the shunt must have a high degree of rigidity. However, if the shunt is made too rigid the bimetal experiences a reverse deflection under extremely high current conditions thereby requiring the shunt to be made of a yieldable material. It can thereby be seen that a compromise must be reached, the end result being that the shunt has less than ideal rigidity and yielda'bility.

In addition to this, the bimetal actuated bimetal shunt is a mechanical operation which consists of the bimetal strip experiencing motion in the direction of the shunt in order to complete the shunting circuit. The deformation of the bimetal is not instantaneously responsive to the heat developed therein and although the shunt may be effective to minimize the deformation which the metaliic shunt experiences, the shunt cannot prevent the destruction of the bimetal due to the rapidly occurring excessive heating.

One method of preventing the undesirable rebounding which takes place between the bimetal and the shunt contacts consists of introducing a magnetic circuit between the bimetal and the bimetal shunt which retains the bimetal and shunt contacts in engagement during the time that overload current is present in the bimetal shunt. A magnetic circuit of this type is set forth in US. application Serial No. 775,862 entitled Thermal Magnetic Current Bypass filed November 24, 1958, now US. Patent No. 3,096,413 by C. E. Gryctko et a1. and assigned to the same assignee of the instant application. In the circuit breaker of the aforesaid application the magnetic means urges the shunt member toward the bimetal upon the occurrence of an overload current. Once the contacts of the shunt path have been engaged the magnetic means exerts a closing force to hold the contacts in engagement and help prevent contact blowotf due to current fiow through these contacts. Although this structure aids in the prevention or" blowoif or rebounding which may occur between the shunt and bimetal contacts, the closing of the shunt circuit remains a mechanical operation thereby requiring a definite interval of time before the shunt contacts are brought into engagement. This magnetic circuit therefore does not overcome the necessity of bringing the shunt circuit into operation instantaneously upon the occurrence of a severe fault current condition.

The device of my invention employs electromagnetic means which presents a high impedance to the shunt circuit during overload conditions and which bypasses the bimctal instantaneously upon the occurrence of a severe fault current condition.

My invention consists of a sensing circuit having two parallel paths one of which has relatively high resistance and low relatively constant reactanec. The remaining parallel path has a relatively low resistance and a widely variable reactance which is so arranged as to decrease in magnitude as the current magnitude increases. When theinput current to the parallel paths is of relatively low magnitude the path having the high magnitude reactance diverts substantially all of the low magnitude input current to the path having the high resistance and the substantially low constant reactance. However, upon the occurrence of a high magnitude current such as, for example, a severe fault current, the parallel path having the initially high magnitude reactance diminishes its reactance instantaneously in response to a predetermined magnitude of fault current, thereby causing substantially all of the fault current to be channelled thereth-rough.

The parallel path having the high resistance and low constant reactance is directly connected to the bimetal trip means of the circuit breaker. With this circuit arrangement it can therefore be seen that the bimetal shunt receives substantially all of the input current during both normal and overload current conditions but upon the occurrence of a severe fault current the parallel path having the widely varying reactance reduces its magnitude of reactance instantaneously upon the occurrence of a severe fault current thereby bypassing the bimetallic element. Since the current bypassing operation occurs instantaneously at a predetermined magnitude of fault current, the sensing circuit protects the bimetal from both distortion and excessive heating. Since the bypass circuit is controlled directly by the bus current magnitude rather than the amount of bimetal deflection the bypass circuit provides more reliable protection against bimetal burnout.

It is therefore one object of my invention to provide a novel bypass circuit for thermal trip means which operates independently of the deflection of the bimetal element.

Another object of my invention is to provide a non mechanical bypass circuit for the protection of a thermal trip means.

Still another object of my invention is to provide a novel overload sensing circuit for the protection of a thermal trip means wherein said sensing circuit includes a current path having a reactance which is inversely proportional to the current flowing therethrough.

Another object of my invention is to provide a bypass circuit which is so arranged as to operate instantaneously upon the occurrence of a predetermined current magnitude.

Still another object of my invention is to provide a novel overload sensing circuit for a circuit breaker having both thermal and magnetic trip means wherein the overload sensing circuit is so arranged as to instantaneously divert the overload current from said thermal trip means to said magnetic trip means at a predetermined current magnitude.

These and other objects will become apparent upon reading the accompanying disclosure and drawings in which:

FIGURE 1 is a side plan view of a circuit breaker employing my novel overload sensing circuit.

FIGURES 2 and 3 are side plan views of other embodiments of the overload sensing circuit shown in FIGURE 1.

Referring now to the drawings, FIGURE 1 shows a molded case circuit breaker 2% having three phases which are electrically isolated from one another by suitable insulating barriers and are mechanically tied together by a common insulating bar 21 and a common tripper bar ZZJ Operating mechanism 23 is immediately connected to center phase [2 (which is the only phase shown in FIGURE 1) and by means of insulating bar 21 and tripper bar 22 the operation of phases a and c (not shown) are coordinated with the operation of phase I) so that all three phases, a through c, are operated simultaneously to the on, off and reset positions. Although FIGURE 1 shows a three phase circuit breaker, it should be noted that my novel overload sensing circuit operates equally as well with one or any number of phases and the three phase circuit breaker set forth herein lends no novelty to the instant invention. 7

Each of the phases a through c includes an identical interruptable current path through circuit breaker 20. For the sake of brevity, only one current path will be descibed, it being understood that the current paths of the other two phases are identical to the current path of the described phase. This current path comprises terminal 25, bus conductor 26 to stationary contact 27. Stationary contact 27 is engageable by movable contact 28 which is mounted on the rigid forward section 29 of contact arm 19. Arm 19 terminates in a rigid portion 31 which is 82 and magnetic core 36 forming the energizing turns therefor and finally terminating in terminal 37. Angularly shaped element 31 is secured to bus conductor and bimetallic element 76 in any well-known process such as brazing or welding. The lower leg 83 of bimetallic element 76 is secured to bus conductor 35 in the same manner as angularly shaped member 31; angularly shaped member 81 and bimetallic element '76 forming a parallel current path around saturable reactor 82 for a purpose to be more fully described, Angularly shaped member 31 and bimetallic element 76' are formed of materials having substantially high resistance relative to the resistance of bus conductor 35 for a purpose to be more fully described.

L-shaped member 80 is suitably secured to bus conductor 35 at one end and to fastener 84 at its other end for the purpose of supporting bus conductor 35 and magnetic core 36.

Breaker operating mechanism 23 comprises toggle linkage 46, 47 connected at the point 43. Linkage 46 is pivotally secured to arm 19 at pin 59. The upper end of linkage 47 is pivotally secured at 52 to latchable cradle 53. Cradle 53 is keyed to shaft 54, the ends of which are journalled in the circuit breaker frame 43. Operating spring d5 is secured at one end to knee 48 and at the other end to handle assembly 56.

Handle assembly 56 comprises a molded member 57 having an upwardly extending portion 58 the end of which projects through an opening in the casing cover (not shown) of the circuit breaker to provide an external operating handle for circuit breaker 2%. Member 57 is rigidly secured to a bifurcated arm 59 which straddles arm 19 and is positionedbetween the walls (not shown) of frame 43. The tips of each bifurcated section are forked and engage pins 69 which extend inwardly from the walls of frame 43.

A hooked tip 61 extends from arm'59 and is in position to engage cradle 53 to cause rotation thereof about pin 54 when operating handle 58 is moved from the tripped to the reset or manual off position. When operating handle 53 is moved to the oif posit-ion tip 62 becomes latched under member d3. Member- 63 is mounted on fixed pivot 64 journalled in bracket 64a and includes the rearwardly extending tip 65 which is latchable on common trip-per bar extension 66. Common "tripper bar 22 is mounted on fixed pivot 67 and biased 'thereabout to a latched position. Tripper bar 22 includes individual extension member 68 for each phase of the circuit breaker 20. Each extension member 68 .is engageable by a plunger 69 which is secured to magnetic armature 7% when the armature 70' is attracted to magnetic core 36 and'serves as an instantaneous trip means for circuit breaker 243. Spring 71' biases armature 78 away from core 36 with the spacing between core 36 and armature 7h being established by adjusting screw 72' which positions pivot member 73' which in turn engages an extension 74 on plunger 69. A screw means 75 is .also mounted to extension 68 and is engageable by himetallic element 76' thereby providing a time delay trip means for circuit breaker 20. If either the time delay trip or instantaneous trip means causes extension 68 to bring about a clockwise rotation of common tripper bar 22 with respect to FIGURE 1, extension 66 will likewise be pivoted in a clockwise direction thus releasing tip 65 and permitting member 63 to be forced counterclockwise about pivot 64, under the influence of operating spring 55, thus freeing tip 62 of cradle 53 and permitting cradle :53 to assume a tripped position.

During periods of normal or overload current flow through each phase of circuit breaker 2t) magnetic core 82 is not in a state of saturation. In this condition magnetic core 82 acts as a very high impedance to the flow of current therethrough causing the main current to be diverted through the path consisting of bimetallic element 76 and angluar member 81 which path does not present an impedance as great as that presented by magnetic core $2.

Since substantially all of the primary current flowing through phase 12 passes through the bypass path of bimetal 76 and member 81, bimetal 76 is directly responsive to current flow acting as a time delay trip means with the tripping action of bimetal 76 being inversely proportional to the bus current magnitude above normal rated current. Although it has been stated that the resistivity of elements '76 and 81 are substantially greater than the resistivity of bus conductor 35, the reactance which magnetic core 32 presents to current flowing through bus conductor 35 results in a higher impedance in bus conductor 35 than is present through the bypass circuit of elements 76' and 81.

At a predetermined current magnitude above normal rated current, magnetic core 82 saturates causing the reactance presented by the core to be instantaneously decreased to the point where its effect upon the current is negligible. At this instant, the impedance of bus conductor 35 consists only of a resistive component which is substantially less than the resistance of the parallel path consisting of bimetallic 76' and angular member 81. Under these conditions, almost all of the fault current passes through bus conductor 35 due to its diminished impedance. Although the parallel path of elements 76 and 81 does carry some percentage of the total current, the magnitude of the current flowing therethrough is considerably less than the amount needed to permanently distort or even overheat bimetal 76'. The saturation of magnetic core 82 is an extremely rapid operation which is performed by the rotational movement of the magnetic domains (not shown) in core 82 until the magnetic poles of the domains are aligned in the same direction.

Prior art devices which operate in response to the movement of a bimetal such as bimetal 75' require substantially longer periods of time due to the substantially large mass of the bimetal in comparison to the mass of each domain in the magnetic core 82.

In addition, magnetic core 82 may be so designed as to saturate at a predetermined current magnitude which will guarantee long operating life of bimetal '76 even after repeated usage.

As long as the resistance impedance ratios of the two parallel paths are kept fairly constant the sensing circiut may be designed to accommodate biemtal elements of varying degrees of sensitivity depending only upon the needs of the particular circuit breaker in which they are to be employed. Since the biemtal 76' is directly heated by the current flow therethrough the design problems or" indirectly heated bimetals utilized in the prior art are completely eliminated.

FIGURE 2 shows the novel sensing circuit wherein a flexible conductor 81 is employed for electrically connecting bimetal 76' to bus conductor 35. The use of flexible conductor 81 permits greater motion of bimetal '76 which guarantees more reliable thermal tripping operations. I

In circuits having very low ratings, multi-turn bus conductors may be employed for current amplification purposes. This is shown in FIGURE 3 Where bus conductor 35 forms a two-turn winding through magnetic core 82 whereas flexible condutcor 81 which is electrically connected to bus conductor 35 at point 86 forms a one-turn winding through magnetic core 82. By designing the sensing or bypass circuit in this manner, the resistance ratios between the parallel turn paths may be regulated by providing a multi-turn bus conductor 35 for magnetic core 82, wherein the resistance ratio between the bimetal current path and the bus conductor current path is regulated by placing the bimetal current path in series with only a small portion of the total turns of multi-turn conductor 35'. During severe current conditions when magnetic core dz becomes saturated the resistivity of flexible conductor 81 and bimetal 76 then controls the 6 amount and distribution of current through the parallel current paths.

It can be seen from the foregoing that I have provided a sensing circuit which instaneaneously serves to protect a circuit breaker thermal trip means automatically upon the occurrence of a predetermined current magnitude.

Although I have here described preferred embodiments of my novel invention, many variations and modifications will now be apparent to those skilled in the art and I therefore prefer to be limited not by the specific disclosure herein but only by the appending claims.

I claim:

1. A device for interruption of AC. current, including a first and a second terminal, a current path between said terminals including a current sensing circuit and circuit means connected in series, said circuit means including cooperating contacts operable into and out of engagement, with said contacts in engagement, said circuit means offering substantially less impedance to current flow than offered by said current sensing circuit, said circuit means having an impedance to current flow which is substantially independent of current magnitude, said current sensing circuit comprising first and second parallel current paths, said first current path having a greater resistance than said second current path, said second current path including impedance means for making the impedance of said second current path substantially greater than the impedance of first current path, the resistance impedance ratio of said parallel current paths providing a predeterminedly controlled current distribution between said parallel current paths during low magnitude current conditions, the impedance of said impedance means being inversely proportional to current flowing therethrough, causing substantially all of the current to be channelled through said first current path during low magnitude current conditions and substantially all of said current to be channelled through said second current path during high magnitude current conditions, said impedance means being a reactor, and means operatively coupled to said first current path and said contacts for opening of the latter responsive to predetermined current conditions in said first current path.

2. A current sensing circuit for interruption of AC. current comprising first and second parallel current paths, said first current path having a greater resistance than said second current path, said second current path including impedance means for making the impedance of said second current path substantially greater than the impedance of first current path, the resistance impedance ratio of said parallel current paths providing a predeterminedly controlled current distribution between said parallel current paths during low magnitude current conditions, the impedance of said impedance means being inversely proportional to current flowing therethrough, causing substantially all of the current to be channelled through said first current path during low magnitude current conditions, and substantially all of said curernt to be channelled through said second current path during high magnitude current conditions, said first current path including means responsive to current passing through said first current path for indicating the current magnitude of said current, said means being adapted to experience deformation, the amount of deformation being proportional to the magnitude of current flowing therethrough.

3. A current sensing circuit for interruption of A.C.

current comprising first and second parallel current paths,

said first current path having a greater resistance than said second current path, said second current path including impedance means for making the impedance of said second current path substantially greater than the impedance of first current path, the resistance impedance ratio of said parallel current paths providing a predeterminedly controlled current distribution between said '4. A bypass circuit for interruption of A.C. current for use in a circuit breaker having a time delay trip means, comprising circuit means including first and second parallel current paths extending part way between 63 variable impedance means wherein said impedance is inversely proportional to the current flowing therethrough, said first current path including a thermal trip member, adapted to be deflected in direct proportion to the current flowing therethrough, said circuit means being adapted to pass substantially all of the current through said first current path when said current is below a predetermined magnitude and to pass substantially the circuit breaker terminals, the resistivity of said first V path being substantially greater than the resistivity of said second path, said second current path including variable impedance means wherein said impedance is inversely proportional to the current flowing therethrough, said first current path including a thermal trip member, adapted to be deflected in direct proportion to the current flowing therethrough, said circuit meansbeing adapted to pass substantially all of the current through said first current path when said current is below a prede termined magnitude and to pass substantially all of the current through said second current path when said current attains said predetermined magnitude. 7

5. A bypass circuit for interruption of, A.C. current for use in a circuit breaker having a time delay trip means, comprising circuit means including first and second parallel current paths extending part way between the circuit breaker terminals, the resistivity of said first path being substantially greater than the resistivity of said second path, said second current path including variable impedance means wherein said impedance is inversely proportional to the current flowing therethrough, said first current path including a thermal trip member, adapted'to be deflected in direct proportion to the current flowing therethrough, said circuit means being adapted to pass'substantially all of' the current through said first current path when said current is below a predetermined magnitude and to pass substantially all of the current through saidsecond current path when said current attains said predetermined magnitude, said impedance means comprising a magnetic core.

6. A bypass circuit forinterruption of A.C. current for use in a circuit breaker having a time delay trip means, comprising circuit means including first and second parallel current paths extending part way between the circuit breaker terminals, the resistivity of said first path being substantially greater than the resistivity of said second path, said second current, path including variable impedance means wherein said impedance is inversely proportional to the current flowing therethrough, 1

said first current path including a thermal trip member,

adapted to be deflected in direct proportion to the current flowing therethrough, said circuit means being adapted to pass substantially all of the current through said first current path when said current is below a predetermined magnitude and to pass substantially all of the current through said second current path when said current attains said predetermined magnitude, said first current path comprising a bimetal element and a conductive member connected in series arrangement, said bimetal element constituting said thermal trip member, said series arrangement being connected in parallel with said second current path. a s

7. A bypass circuit for interruption of A.C. current for use in a circuit breaker having a, time delay trip means, comprising circuit means including first and secondparallel current paths extending part way between :said second path, said second current path including all of the current through said second current path when said current attains said predetermined magnitude, sald first current path comprising a bimetal element and a conductive member connected in series arrangement,

said bimeta'l element constituting said thermal trip memher, said series arrangement being connected in parallel with said second current path, said conductive member being flexible to permit maximum deflection of said bimetal element. 7 r

8. A bypass circuit for interruption of A.C. current for use in a circuit breaker having a time delay trip means, comprising circuit means including first and second parallel'current paths extending part way between the circuit breaker terminals, the resistivity of said first path being substantially greater than the resistivity of said second path, said second current path including variable impedance means wherein said impedance is inversely proportional to 'the current flowing therethrough, said first current path including a thermal trip member, adapted to be deflected in direct proportion to the current flowing therethrough, said circuit means being adapted to pass substantially all of the current through said first current path when said current is below a predetermined magnitude and to pass substantially all of the current through said second current path when said current attains said predetermined magnitude, said second current path comprising a conductive member and a magnetic core, said conductive member being threaded through said magnetic core.

, 9., A bypass circuit for interruption of A.C. current for use in a circuit breaker having a time delay'trip means, comprising circuit means including first and second parallel current paths extending part way between the circuit breaker terminals, the resistivity of said first path being substantially greater than the resistivity of said second path, said second current path including variable impedance means wherein said impedance is inversely proportional to the current flowing therethrough, said first current path including a thermal trip member, adapted to be deflected in direct proportion to the current'flowing therethrough, said circuit means being adapted'to pass substantially all of the current through said first current path when said current is below a predetermined magnitude and to pass substantially all of the current through said second current path when said current attains said predetermined magnitude, said second current path comprising a conductive member and a magnetic core, said conductive member being threaded through said magnetic core, to form a'plurality of turns through said magnetic core, said first'current path comprising a bimetal element and a conductive member connected in series arrangement, said series arrangement be-' I ing connected in parallel with said second current path.

variable impedance means wherein said impedance is inversely proportional to the current flowing therethrough, said firstscurrent path including a thermal trip member, adapted to be deflected in direct proportion to the current flowing therethrough, said circuit means beingadapted to pass substantially allot the current through said first current path when said current is below a predeterrnined magnitude and to pass substantially all of the current thrcughsaid second current path when said current attains said predetermined magnitude, said second current path comprising a conductive member and a magnetic core, said conductive member being threaded through said magnetic core, to form a plurality of turns through said magnetic core, said first current path comprising a bimetal element and a conductive member connected in series arrangement, said series arrangement being connected in parallel with some of said turns and being connected in series with at least one turn of said second current path.

11. A circuit breaker for interruption of A.C. current including a pair of cooperating contacts and first means operably connected to one of said cooperating contacts, an arm carrying one contact of said pair of cooperating contacts; said arm being operable to a first and a second position wherein said cooperating contacts are disengaged and engaged respectively; said first means being biased to urge said contacts towards disengagement, normally closed latch means which when unlatched prevents said first means from closing said cooperating contacts, current sensing means responsive to the current flowing through said cooperating contacts for unlatching said latch means during overload current conditions, static second means circuit connected to the current flowing through said cooperating contacts and electrically varying responsive to said current flow for diverting substantiallly all of said current away from said current sensing means upon the occurrence of a predetermined current magnitude and for diverting substantially all of said current away from said second means when said current is below said predetermined magnitude.

12. A circuit breaker for interruption of AC. current including a pair of cooperating contacts and first means operably connected to one of said cooperating contacts, an arm carrying one contact of said pair of cooperating contacts; said arm being operable to a first and a second position wherein said cooperating contacts are disengaged and engaged respectively; said first means being biased to urge said contacts towards disengagement, normally closed latch means which when unlatched prevents said first means from closing said cooperating contacts, current sensing means responsive to the current flowing through said cooperating contacts for unlatching said latch means during overload current conditions, static second means circuit connected to the current flowing through said cooperating contacts and electrically varying responsive to said current flow for diverting substantially all or" said current away from said current sensing means upon the occurrence of a predetermined current magnitude and for diverting substantially all said current away from said second means when said current is below said predetermined magnitude, said second means including a magnetic core having at least one winding the impedance of said second means varying responsive to the current flow through said winding.

13. A circuit breaker for interruption of AC. current including a pair of cooperating contacts and first means operably connected to one of said cooperating contacts, an arm carrying one contact of said pair of cooperating contacts; said arm being operable to a first and a second position wherein said cooperating contacts are disengaged and engaged respectively; said first means being biased to urge said contacts towards disengagement, nor- ,aonsss mally closed latch means which when unlatched prevents said first means from closing said cooperating contacts, current sensing means responsive to the current flowing through said cooperating contacts for unlatching said latch means during overload current conditions, static second means circuit connected to the current flowing through said cooperating contacts and electrically varying responsive to said curernt flow for diverting substan tially all of said current away from said current sensing means upon the occurrence of a predetermined current magnitude and for diverting substantially all of said current away from said second means when said current is below said predetermined magnitude, said second means including a magnetic core having at least one winding, the impedance of said second means varying responsive to the current flow through said winding, said current sensing means comprising a bimetal element, said bimetal element being electrically connected across the winding of said magnetic core.

14. A circuit breaker for interruption of A.C. current including a pair of cooperating contacts and a first means operably connected to one of said cooperating contacts, an arm carrying one contact of said pair of cooperating contacts said arm being operable to a first and a second position wherein said cooperating contacts are disengaged and engaged respectively; said first means being biased to urge said contacts towards disengagement, normally closed latch means which when unlatched prevents said first means from closing said cooperating contacts, current sensing means responsive to the current flowing through said cooperating contacts for unlatching said latch means during overload current conditions, static second means circuit connected to the current flowing through said cooperating contacts and electrically varying responsive to said current flow for diverting substantially all of said current away from said current sensing means upon the occurrence of a predetermined current magnitude and for diverting substantially all of said current away from said second means when said current is below said predetermined magnitude, said second means including a magnetic core having at least one winding, the impedance of said second means varying responsive to the current flow through said winding, said current sensing means comprising a bimetal element, said bimetal element being electrically connected across the winding of said magnetic core, said electrical connection between said bimetal and said second means being so arranged as to place said bimetal in the conductive path thereby avoiding the necessity for a heater element, current flowing through said bimetal element generating substantially the total heating energy for said bimetal element.

References Cited by the Examiner UNITED STATES PATENTS 1,540,307 6/25 Beall 317-16 2,989,606 6/61 Walker et al. 200-88 3,049,609 4/62 Purkhiser et a1 2l9-131 3,073,925 1/63 Thomas et al. ZOO-88 3,141j081 7/64 Cellerini 200-88 BERNARD A. GILHEANY, Primary Examiner.

MAX L. LEVY, Examiner. 

1. A DEVICE FOR INTERRUPTION OF A.C. CURRENT, INCLUDING A FIRST AND A SECOND TERMINAL, A CURRENT PATH BETWEEN SAID TERMINALS INCLUDING A CURRENT SENSING CIRCUIT AND CIRCUIT MEANS CONNECTED IN SERIES, SAID CIRCUIT MEANS INCLUDING COOPERATING CONTACTS OPERABLE INTO AND OUT OF ENGAGE MENT, WITH SAID CONTACTS IN ENGAGEMENT, SAID CIRCUIT MEANS OFFERING SUBSTANTIALLY LESS IMPEDANCE TO CURRENT FLOW THAN OFFERED BY SAID CURRENT SENSING CIRCUIT, SAID CIRCUIG MEANS HAVING AN IMPEDANCE TO CURRENT FLOW WHICH IS SUBSTANTIALLY INDEPENDENT OF CURRENT MAGNITUDE, SAID CURRENT SENSING CIRCUIT COMPRISING FIRST AND SECOND PARALLEL CURRENT PATHS, SAID FIRST CURRENT PATH HAVING A GREATER RESISTANCE THAN SAID SECOND CURRENT PATH, SAID SECOND CURRENT PATH INCLUDING IMPEDANCE MEANS FOR MAKING THE IMPEDANCE OF SAID SECOND CURRENT PATH SUBSTANTIALLY GREATER THAN THE IMPEDANCE OF FIRST CURRENT PATH, THE RESISTANCE IMPEDANCE RATIO OF SAID PARALLEL CURRENT PATHS PROVIDING A PREDETERMINEDLY CONTROLLED CURRENT DISTRIBUTION BETWEEN SAID PARALLEL CURRENT PATHS DURING LOW MAGNITUDE CURRENT CONDITIONS, THE IMPEDANCE OF SAID IMPEDANCE MEANS BEING INVERSELY PROPORTIONAL TO CURRENT FLOWING THERETHROUGH, CAUSING SUBSTANTIALLY ALL OF THE CURRENT TO BE CHANNELLED THROUGH SAID FIRST CURRENT PATH DURING LOW MAGNITUDE CURRENT CONDITIONS AND SUBSTANTIALLY ALL OF SAID CURRENT TO BE CHANNELLED THROUGH SAID SECOND CURRENT PATH DURING HIGH MAGNITUDE CURRENT CONDITIONS, SAID IMPEDANCE MEANS BEING A REACTOR, AND MEANS OPERATIVELY COUPLED TO SAID FIRST CURRENT PATH AND SAID CONTACTS FOR OPENING OF THE LATTER RESPONSIVE TO PREDETERMINED CURRENT CONDITIONS IN SAID FIRST CURRENT PATH. 